them stranded on land. Some former islands such as Porsön and Hertsön near the city of Luleå are still called islands, but are now connected to the mainland. The maximum depth at the Kvarken sound today is around . In not less 2,000 years the exit from the bay at Kvarken will be raised above sea level, which will result in it becoming Europe's largest lake. Hydrology The Bothnian Bay has a harsher environment than other parts of the greater Baltic Sea. The bay is ice-covered for 110 to 190 days each year. Tides have little effect, but high winds driving the water from the south or north may cause the water level to rise or fall by . Major rivers that flow into the bay include: The salinity is only about 0.2 psu in the northern part of the bay, dropping almost to zero in some of the archipelagos with large river inflows. The low salinity and cold temperatures in winter results in ice that is considerably stronger than more saline or warmer ice. Islands If an island is defined as an area of land more than that is surrounded by water, the Bay of Bothnia has 4,001 islands. The largest island is Hailuoto. The north of the bay contains a large archipelago area. The Swedish portion of this area is the Norrbotten archipelago. This is divided into the Piteå, Luleå, Kalix and Haparanda archipelagos. Many of the islands are uninhabited and in a natural state. In the winter the larger islands may be accessed via ice roads. Some of them are inhabited or have seasonal fishing villages used by people from the mainland. The Swedish Haparanda Archipelago National Park () occupies the Haparanda group of islands, bordering the Finnish Bothnian Bay National Park. It includes the larger islands of Sandskär and Seskar Furö, and some smaller islands and skerries. All of these islands have emerged in the last 1,500 years as the bed of the bay has risen. The Bay of Bothnia National Park in the Finnish section (, ), established in 1991, is located in the archipelago offshore from Tornio and Kemi. It covers of which about is land. Climate The immediate coastal areas of the Bothnian Bay tend to be bordering between humid continental and subarctic climates. The summers on the coastlines are the northernmost coastlines to average above in summer. In winter the sea freezes